The so-called fuel cell that extracts the energy obtained from the electrochemical reaction between hydrogen and oxygen as electric power, is anticipated to be used widely in various kinds of applications such as portable equipment and automobiles. Generally, this fuel cell ensures substantial power and is made by laminating several tens to several hundreds of unit cells of basic configuration (hereinafter referred to as “unit cells”) in series, each unit cell having a membrane electrode assembly (hereinafter referred to as “MEA”) provided with electrode and a gas-diffusion layer, such as carbon paper on both surfaces of the electrolytic film, with at least fuel (such as hydrogen gas), oxidizing agent (air or oxygen) or cell on one face sandwiched between two bipolar plates on which coolant flow passages for cooling cells are formed.
Accordingly, “conductivity” for increasing the power generating efficiency of the fuel cell is required in the bipolar plates used in these fuel cells, and at the same time, the demand for “thinness” of bipolar plates is high because of the need for miniaturization of the fuel cell. Also, as mentioned above, for obtaining substantial power, the fuel cell generally makes use of a plurality of bipolar plates laminated in the thickness direction, Therefore, the bipolar plate itself is to be provided with high “thickness precision,” the contact resistance between bipolar plate and MEA, and between fuel cells is to be reduced, and air tightness and watertightness in the packing and gaskets for various kinds of gas and liquid seals introduced in the fuel cell is anticipated to be ensured. From these aspects, a molding material with high conductivity, very thin and having high thickness precision suitable for production of bipolar plates is demanded. Moreover, from economic aspects, a method of production of bipolar plates at low cost and with high productivity is demanded.
In view of these reasons, after mixing conducting agent and thermoplastic resin, sheet forming material was made by a conventional method such as the extrusion molding method or the hot rolling method, and a method of molding this sheet molding material into the specified bipolar plate using a mold provided with flow passages for fuel and/or oxidizing agent has been proposed (for example, see patent documents 1 and 2).
However, during the kneading step of thermoplastic resin and conductive particles of graphite, for instance, which is used as conducting agent, and during the extrusion step of this mixture in these methods, strong shear force and compressive force are applied on the thermoplastic resin and the conducting agent. As a result, the conductive particles are converted to powder, the number of conductive particles increases, the contact resistance between the conductive particles increases, and the conductivity of the bipolar plate obtained by molding the sheet material decreases, which is a problem.
Moreover, if the ratio of conducting agent in the mixture is increased to above 80% by weight with the objective of improving the conductivity of the bipolar plate, larger shear force and compressive force are necessary in the kneading step, sheet making step and molding step of the mixture mentioned above. As a result, the conductive performance desired in the bipolar plate is difficult to achieve, and since conducting agent with high concentration is included, the workability becomes poor, and thin sheets cannot be obtained easily. Furthermore, a bipolar plate such as the one obtained by molding this sheet material has problems such as poor transferability of mold shape, defects in dimensional precision can occur easily, and thickness precision also becomes an issue.
Patent document 3 proposes a method of applying the so-called conductive paint dispersed uniformly with conductive fine particles such as graphite in epoxy resin on a non-woven fabric surface, as a method of obtaining conductive thin sheet without applying strong shear force or compressive force to the conducting agent.
However, according to this method, to uniformly apply a coat of epoxy resin with conductive fine particles uniformly dispersed therein on a non-woven fabric, the amount of conductive particles to be added must be reduced to about 35% to 60% and flowability must be ensured; with this amount of conductive particles, a conductivity of less than 200 mΩ·cm required for the bipolar plate for fuel cell cannot possibly be achieved.
Accordingly, high conductivity required for the bipolar plate for fuel cells was available in the conventional molding methods by sheet stamping, sheet rolling and blanking, but it was difficult to manufacture molding material suitable for production of thin bipolar plate with high thickness precision.
Also, a bipolar plate for fuel cell (for example, refer to patent document 4) obtained by heating and softening non-woven fabric including thermoplastic resin fiber of diameter 0.1 to 20 μm with conductive particles distributed uniformly therein, and molding it in the mold has been proposed. In this method, although a sheet molding material with a thickness of about 0.05 mm is obtained, since conductive particles are distributed within the non-woven fabric, thin sheet molding material thinner than this sheet cannot be obtained; thus, very thin bipolar plates cannot be obtained. Moreover, a step of making non-woven fabric once using thermoplastic resin fabric and conductive particles as raw materials has to be included; this results in disadvantages with regard to production efficiency, and furthermore, the thickness precision of the non-woven fabric thus obtained tends to be poor.    Patent document 1:    Japanese Unexamined Patent Application, First Publication No. 2001-122677    Patent document 2:    Japanese Unexamined Patent Application, First Publication No. 2002-198062    Patent document 3:    Japanese Unexamined Patent Application, First Publication No. 2003-89969    Patent document 4:    Japanese Unexamined Patent Application, First Publication No. 2004-356091